In relation to some diseases patients must inject a medicament on a regular basis such as once weekly, once daily or even a plurality of times each day. In order to help patients overcome fear of needles, fully automatic injection devices have been developed that makes the use of an injection device as simple as possible. Such devices are typically designed such that a user shall position the injection device onto the injection site and activate the device. Such activation causes the device to insert a needle into the skin, eject a dose of the medicament and subsequently move the needle into a shielded position.
Generally, for injection devices of the above type, main attention has been directed towards devices equipped with a glass cartridge where a needle cannula is fixedly attached to the outlet end of a cartridge. Such needle cannula is initially being covered in a sterile way by a cap member that during storage acts as a stopper for the needle cannula, and which requires removal prior to use. Typically, these devices further include a needle shield portion for shielding the needle before and/or after use. Disclosure of such devices is included in U.S. Pat. No. 7,449,012, U.S. Pat. No. 7,717,877 and WO2008/116688.
Some manufacturers prefer the type of cartridge having a pierceable septum which during storage provides a seal for the cartridge outlet and where the septum, upon use, is pierced by a needle cannula. Prior art devices using this type of cartridge are disclosed in U.S. Pat. No. 2,752,918, U.S. Pat. No. 5,658,259, U.S. Pat. No. 6,743,203, U.S. Pat. No. 6,210,369 and WO94/07553. Devices of that type hold a needle assembly and a cartridge in a separated storage configuration which upon activation of the device allows for subsequent connection to establish fluid communication between cartridge and needle assembly. In addition, automatic penetration of the needle into the skin of the user for subsequent automatic delivery of the medicament is typically incorporated.
While the above devices aim at providing a high level of automation, injection devices that provide automatic insertion of the needle into the dermis also prevent the user from controlling the insertion, which can lead to uneasiness for the user. In WO2008/116688, whilst providing manual control for insertion of the needle by means of a needle shield which is moved relative to the needle, the disclosed type of device still utilize the type of cartridge having an integrated needle cannula.
Injection devices that provide automatic delivery of the medicament, i.e. auto-injectors, typically use a spring as driving force for the injection. Before use, the driving spring will be held in a pre-tensioned position from which it is released upon activation of the device. After activation the spring uses the energy from the tension to drive forward the piston of a cartridge.
One problem associated with auto-injectors is that the piston is moved forward, the tension of the spring decreases which in turn decreases the force of the spring pressing on the piston. When using auto-injectors it is important to have a clear feedback communicating to the user when the injection is finished in order to prevent the user from removing the device from the injection site prematurely. WO 2010/035057 and WO 2010/035059 disclose auto-injectors that provide an audible end of dose confirmation. However, these solutions are somewhat problematic in that the energy used to provide the audible feedback is required near the end of the injection where the spring force available for driving the injection is at a minimum. This sets a limit for the amount of energy which can be used to provide feedback to the user as the dosing force and friction forces in the cartridge/syringe also must be overcome.
A different approach is disclosed in WO 2005/070481 wherein the device may include two arms that are pre-tensioned during assembly of the device and where the pre-tensioned arms are released momentarily when the device enters the end of dose condition to generate a click sound. However, devices wherein parts remain in a tensioned state during storage generally leads to creep of the material which generally results in a non-optimal performance.
Still other problems are associated with auto-injectors that use a medicament cartridge and a needle assembly that during storage is kept in a separated configuration. Connecting the cartridge and needle is in many cases done by using the dose mechanism to move the cartridge forward thus allowing the needle to penetrate the sterile barrier of the cartridge. As the dose mechanism often comprises a pre-tensioned spring this means that the initial relaxation of the spring is used to move the cartridge in contact with the needle. As the spring must be able to overcome forces acting against it while ejecting the medicament, e.g. friction, the spring force needs to be sufficiently high to move the plunger within the cartridge even near the end of the dosing movement. As the pre-tensioned spring provides the highest force at the start of the dose, this means that the highest force provided by the spring is used to move the cartridge into contact with the needle. Moving the cartridge into contact with the needle requires considerably less force than what is required for ejecting the medicament, which means that the cartridge is subjected to a high force and therefore moved into contact with the needle at a high speed. As the cartridge reaches its contact point with the needle it is stopped mechanically. Because of the high speed of movement this stop will emit a considerable amount of noise which can be heard by the user.
This sound can cause uneasiness with the user, thus resulting in decreased confidence toward the product. US2007/0219498 includes disclosure of a device which includes a stationary shock absorber to reduce dynamic stresses on internal components.
Having regard to the above-identified prior art devices, it is an object of the present invention to provide an injection device which enables improved control of the device during operation.
A further object of the invention is to provide an injection device, which provides an improved end of dose indication feature.
A further object of the invention is to provide an injection device which performs more quietly during the initial stages of operation.
Yet additional further objects of the invention are to provide measures for obtaining devices having a superior performance and, at the same time, enabling manufacture at a reduced cost.